israel sealed off the main roads around gaza. will israel invade on the ground? >> i think the chances are going up. >> reporter: jeffrey white, a former analyst with the defense intelligence agency, says an israeli ground invasion of gaza would be a brutal, bloody grind. >> there's a pretty high density of population throughout the strip. it's highest in the major areas, rafa, gaza city, but there are a lot of civilians in other places as well. the other part of this is hamas fights from inside the cities. >> reporter: cities of narrow streets, bazaars, apartment buildings. translation, a punishing building-to-building slog, in a place that's slightly twice the size of washington, d.c. we used a google map with cnn contributor, james spyder marcus. >> what kind of combat are we talking about here? >> this is called combat in restricted terrain. and clearly what we have here in gaza city, there are about 500,000 people that live in this city. and you can only imagine the type of combat that's got to take place in this very restricted terrain. >> reporter: terrain